The Department of Justice has suggested it’s willing to use what observers have called the "nuclear option" in its battle with Apple over the iPhone’s security measures: demanding that Apple turn over its products’ source code as well as the signing key that validates software as coming from Apple, reports Reuters.

A product’s source code is the most fundamental underlying part of any software. With access to it, anyone can change or manipulate any aspect of a product’s software. A signing key validates software as being from a legitimate, authorized source. Devices use a signing key to determine if software is safe to install. If the DOJ were able to force Apple to turn over its source code and signing keys to its software, it could theoretically make its own version of iOS that it could then install on any iOS device in the world without users being able to tell the difference.

In what observers have called a carefully calibrated threat, reports Reuters, the DOJ suggested it could be willing to go so far as requesting Apple’s source code in a footnote in its latest court filings last week, in which it rejected Apple’s arguments for its unwillingness to comply with a court order that ruled Apple must help the FBI bypass iOS’s security measures to access the iPhone used by one of the suspects in the San Bernardino shooting.

In the footnote DOJ lawyers said they have so far refrained from requesting the source code to iOS because they thought "such a request would be less palatable to Apple. If Apple would prefer that course, however, that may provide an alternative that requires less labor by Apple."

Technology, privacy, and security experts who spoke to Reuters said that if the DOJ could successfully order Apple to turn over iOS’s source code then other foreign governments would demand the same thing. The experts called the request the government’s "nuclear option" and said it was a "terrible" idea.

But it appears Apple isn’t taking the DOJ’s source code threat too seriously. Reuters reports that "someone on Apple’s side" revealed Apple isn’t even worried enough about the threat to counter it in its court brief, which is due today. Proceedings in the court case between Apple and the FBI will officially begin next Tuesday, March 22nd.